The present invention relates to detecting a registration mark associated with an object, and more particularly to detecting a registration mark representing a binary code sequence using matched filtering.
A vast number of applications exist in which it is necessary or desirable to monitor certain parameters related to an object. One common example is the need to monitor the position, velocity, acceleration, etc. of one or more components in a manufacturing process. For instance, in a largely automated process for manufacturing disposable diapers, certain components (e.g., absorbent pads, waist elastic bands, printed graphics, etc.) must be positioned or aligned with respect to other components (e.g., support layers, etc.) in order to produce an acceptable product. To facilitate this process, registration marks are commonly applied to certain components. These registration marks are then detected during the manufacturing process using sensors to determine parameters of interest including, for example, when a component is present at a particular location, where to position a particular component, etc.
Frequently, a registration mark is applied to an object as a single mark which, when detected by a sensor, produces a single pulse at the sensor output. This approach, however, sometimes results in detection errors. For example, noise in the system may falsely trigger the sensor output, or may prevent detection of the registration mark. Additionally, sensors may confuse one registration mark with another similar mark. Regardless of the cause, these detection errors can all lead to errors in the registration process and component positioning during product manufacture. As a result, the quality of the manufactured product may suffer, and the product itself may have to be discarded at a corresponding cost to the manufacturer.
It is also known to use registration marks which represent a specific binary code sequence referred to as a xe2x80x9cperfect word.xe2x80x9d Perfect words are known to exhibit autocorrelation functions with low sidelobes when used in certain radar applications. In one registration system using perfect words, a matched filter compares each bit in a detected sequence of bits with corresponding bits in the perfect word. The filter output is then incremented by one for each matching pair of compared bits. Thus, in the case of a seven bit perfect word, the filter output will have a maximum amplitude of seven upon detecting a sequence of bits which completely matches the perfect word. As best understood, however, no change is made to the filter output in response to a non-matching pair of compared bits. Therefore, if all but one pair of compared bits match, then the filter output will have an amplitude of six (assuming a seven bit perfect word is used). Thus, the difference in the filter output for matching and non-matching inputs may be as small as one, as is the case in the single pulse registration systems described above.
As recognized by the inventors hereof, what is needed is a registration system which produces a filter output having a high amplitude in response to a matching input, and an advantageously lower amplitude, as compared to the prior art, for non-matching inputs.
In order to solve these and other needs in the art, the inventors hereof have succeeded at designing a system and method for detecting a registration mark associated with an object which provides improved resolution between matching and non-matching inputs. The registration mark preferably represents a binary code sequence of N bits, where N is an integer greater than one. Upon detecting a sequence of N bits with a sensor, the detected sequence is provided to a code matching filter, which compares the detected sequence with the binary code sequence and produces a detection signal representing the results of this comparison. Importantly, the code matching filter preferably increases a value of the detection signal for each matching pair of compared bits, and decreases the value of the detection signal for each non-matching pair of compared bits. In this manner, a penalty is assessed for mismatches. This results in a filter output having a high amplitude in response to a matching input, and a lower amplitude, as compared to the prior art, for non-matching inputs. For improved response, the binary code sequence is preferably a Barker code and, even more preferably, is a Barker code having roughly the same number of positive and negative bits. The present invention also provides a system for sampling and filtering a detected sequence of bits in such a manner as to avoid potential edge timing problems.
In accordance with one aspect of the present invention, a method is provided for detecting a registration mark associated with an object, where the registration mark represents a binary code sequence of N bits with each bit having one of two discrete values, and where N is an integer greater than one. The method of detecting the registration mark includes comparing N bits of a sensor signal with corresponding bits of the binary code sequence, and generating a detection signal in response to the comparing, including increasing a value of the detection signal for each matching pair of compared bits and decreasing the value of the detection signal for each non-matching pair of compared bits, the detection signal indicating detection of the registration mark when the detection signal exceeds a predefined value.
In accordance with another aspect of the present invention, a system is provided for detecting a registration mark associated with an object, where the registration mark represents a binary code sequence of N bits with each bit having one of two discrete values, and where N is an integer greater than one. The system includes a shift register configured to receive a sequence of N bits from a sensor, and a matching filter operatively connected to the shift register. The matching filter is configured to compare the sequence of N bits received by the shift register with the binary code sequence, and to generate a detection signal in response to the comparing. The matching filter is also configured to increase a value of the detection signal for each matching pair of compared bits, and to decrease the value of the detection signal for each non-matching pair of compared bits. The detection signal indicates detection of the registration mark when the detection signal exceeds a predefined value.
In accordance with yet another aspect of the invention, a method of detecting a sequence of bits having a known pattern includes sampling each bit of an input bit sequence at least twice to produce at least two versions of the input bit sequence, comparing each produced version of the input bit sequence to a reference bit sequence, and generating a detection signal in response to the comparing.
While some of the principal features and advantages of the invention have been described above, a greater and more thorough understanding of the invention may be attained by referring to the drawings and detailed description of preferred embodiments which follow.